What usually serves as the seal arrangement for a sensing element of oxygen probes is a precompressed and presintered powder packing made of steatite, which during assembly is exposed to a high axial pressure and is thereby reshaped. The sealing effect of this principle with respect to exhaust gases is entirely sufficient, but fuel in liquid or vapor form, and water vapor, can diffuse through the seal element and thereby interfere with probe operation.
German Patent No. 195 32 090 describes a seal arrangement which improves the seal effect with respect to fuel vapor by using a so-called "sandwich packing" of seal elements, in which a boron nitride disk placed in the packing decreases permeability for hydrocarbons. This solution is expensive due to the use of boron nitride.
A further alternative would be fusible sealing of the sensing element. The development of this approach requires extremely precise matching of the coefficients of thermal expansion of the materials used, which can lead to problems in large-volume production.